Varistors are devices having non-ohmic properties in which current rapidly increases with increase in voltage. The varistors are used for the purpose of protecting electric circuits from surge voltage by use of such non-ohmic properties. Specifically, a varistor is connected in parallel to equipment to be protected. As a result, the varistor keeps insulation against an earth under normal voltage, whereas upon application of surge voltage to an electric circuit, the varistor can dissipate current to an earth without elevating voltage at both ends thereof, and therefore prevents the voltage of the electric circuit from exceeding the withstand voltage of the equipment to be protected.
Various materials for varistors have heretofore been known, and one example thereof includes ceramic obtained by adding metal (e.g., Bi, Sb, Co, or Mn) oxide to a main component ZnO (zinc oxide) and firing the mixture. This material is a polycrystal composed of main-phase zinc oxide particles and a grain boundary layer composed mainly of bismuth oxide, and has a fine structure where spinel particles made of Zn and Sb are irregularly scattered in the polycrystal. Transition metal (e.g., Co or Mn) oxide is present in a solid solution form in the main phase, the grain boundary layer, and the spinel particles. In this material, current passes through the interface between the zinc oxide particles and the grain boundary layer, thereby exhibiting non-ohmic properties. Such a material, however, has the disadvantages attributed to ceramic that the material has a low degree of molding freedom and also has low impact resistance because of being rigid and fragile.
Thus, studies have been made on the preparation of a non-ohmic resistor having a high degree of molding freedom and high impact resistance by kneading a resin material with a varistor powder (microvaristor). Patent Literature 1 describes a non-ohmic resistor containing an insulating matrix based on a polymer such as an epoxy resin, and a filler in a powder form embedded in the matrix. In this literature, the filler used is obtained by mixing particles of a microvaristor having non-ohmic properties with conductive particles, followed by heat treatment to bond the conductive particles contacted with the surface of the microvaristor to the microvaristor. In Patent Literature 1, nearly spherical particles made of the ceramic mentioned above are used in the microvaristor. Also, particles (e.g., made of Ni) that have a smaller size and higher conductivity than those of the microvaristor and have a high-aspect ratio shape, such as a plate, flake, or short fiber shape, which is easy to contact with the surface of the microvaristor are used as the conductive particles.
In this non-ohmic resistor, in order to form a current pathway, the number of microvaristors is increased, while two microvaristors differing in particle size are used so that the smaller microvaristor particles enter into the gaps in the larger microvaristor particles, thereby increasing the density of the microvaristor particles. The thus-increased density of the microvaristor particles increases the number of points of contact between the microvaristor particles (via the conductive particles). As a result, the current pathway is formed, thereby exhibiting non-ohmic properties.
Since this non-ohmic resistor contains a matrix based on a polymer, a component (e.g., switch) of electrical equipment can be used in itself as a varistor by coating the surface of the component with the non-ohmic resistor by a casting method or painting the surface with the non-ohmic resistor.